The present invention pertains to cored wires for treating molten metals to remove unwanted impurities and, in particular, to the manufacture and use of cored wires having a reactive metal core.
The beneficial aspects of calcium addition to steel have been well known for the purposes of inclusion modification. Various techniques have been used to introduce the calcium into the molten steel bath in a cost effective manner including the addition of bulk alloy such as calcium silicon, the powder injection of various alloys and mixtures of calcium metals and the use of wires containing mixtures of calcium and other powders. These techniques have been successful in many instances and the usage of calcium and calcium alloys have become common practice in the manufacture of ferrous metals.
Cored wires, in particular a calcium core surrounded by a steel sheath or jacket, have found wide application in the treating of molten ferrous metals. The cored wire is used to introduce calcium into the molten ferrous metal, after the metal is tapped from a furnace, in order to reduce unwanted elements such as sulfur and oxygen in the molten bath and to control the size and shape of inclusions in the solidified metal. A detailed discussion of the overall process of using such wire is contained in U.S. Pat. No. 4,481,032, the specification of which is incorporated herein by reference.
However, due to the metallurgical properties of calcium, including a high vapor pressure and low melting and boiling points, addition of calcium to a molten steel bath presents problems. Powder injection of calcium powder or alloys of calcium mixed with various fluxes and other materials is practiced in some plants but the technology is expensive, the results are inconsistent and the equipment requires a significant amount of space in the users plant. Furthermore, powder injection of calcium is difficult to apply in a cost effective manner.
In order to overcome the problems with the use of calcium powder the steel clad solid calcium cored wire was developed as a solution to the problems encountered by powder injection. U.S. Pat. Nos. 4,035,892, 4,097,268 and 3,915,693 provide a good background discussion of the use of cored wires wherein a granular material or a mixture of granular materials such as calcium and silicon are encased in a steel wire in order to introduce the calcium or calcium silicon into the molten ferrous metal bath. The calcium can be injected into the molten bath as a surface fed wire or by injection through a gas purged refractory lance such as discussed in the ""032 patent noted above and U.S. Pat. Nos. 4,705,261 and 4,512,800. With these techniques the calcium core is either a solid metallic calcium rod, calcium particles or a mixture of calcium particles with varying amounts of iron powder and/or aluminum powder.
The aluminum powder is added to reduce the vapor pressure of the calcium metal resulting in a more reproducible calcium recovery and less reactivity and splashing when the mixture is added to the steel. However, when using a particulate core, even with a mixture of aluminum powder and calcium, problems still exist. Due to the hydroscopic and reactive nature of metallic calcium it has a limited shelf life and is prone to surface oxidation. In addition powdered metals are dangerous to handle, and the filling of the steel jacket is prone to non-uniform fill rates due to different powder diameters and morphologies, resulting in wire that is expensive to make and difficult to use.
In one method of manufacture, a calcium metal core is extruded into an elongated shape or wire which has a generally cylindrical cross-sectional shape. The core wire is inserted into a metallic sheath or jacket, e.g. steel, the sheath formed as it is continuously roll formed into a tube. The tube is formed with a mechanical lock seam so that reactive metal, e.g. calcium, is encapsulated or locked inside. The resulting structure or product is a continuous tube or wire, being a composite of a reactive core and a roll formed metallic sheath, or jacket. One of the problems with the prior art roll forming process was the insertion of the core into the metallic sheath during the roll forming process. This problem has been addressed in co-pending U.S. patent application Ser. No. 09/000,990 filed Dec. 30, 1997, the specification which is incorporated herein by reference.
Thus in its broadest aspect the present invention relates to fabrication of a cored wire for introducing reactive metals into a molten metal bath by fabricating the cored wire with an outer jacket having a higher melting point than an inner core material, the inner core material being a composite of a first reactive metal surrounded by a sheath of a second reactive metal, the first and second reactive metals melting at lower temperatures than the outer jacket to form an alloy prior to melting of the outer jacket. The composite inner core can include a third layer of yet another reactive metal or a composite of two or more reactive or reactive and non-reactive metals as the second layer.
It has been discovered that encapsulating a solid calcium rod or wire in an aluminum jacket prior to insertion into the steel jacket or sheath results in an improved cored wire and an improved method of introducing the wire into a molten ferrous metal bath. Therefore, in one aspect the present invention is a cored wire for introducing calcium and aluminum into a bath of molten metal produced by: extruding the calcium metal into an elongated wire having a generally cylindrical shape; covering the calcium wire with a sheath of aluminum to form a composite core wire; and inserting the composite core wire into a steel jacket.
In another aspect the present invention is a method of treating molten ferrous metal with calcium metal comprising the steps of: providing a cored wire consisting essentially of an inner core of calcium wire surrounded by a jacket of aluminum, to form a component wire core which is covered by a steel jacket; and introducing the cored wire as a continuous structure into a bath of molten ferrous metal until a desired weight of calcium has been introduced into the molten ferrous metal.
In yet another aspect the present invention is a method for reducing splashing and reactivity of calcium metal when introduced into a bath of molten ferrous metal as a calcium wire surrounded by a steel jacket comprising; the step of forming a core composite of calcium wire covered with a jacket of aluminum, followed by insertion of the core composite into the steel jacket.